1. Introduction
This invention relates to reduced catalytic metals having a mean particle size not exceeding about 500 Angstroms and more particularly, to adsorbable catalytic compositions useful for electroless metal deposition, to methods for making said catalytic adsorbates and to methods of using the same.
2. Description of the Prior Art
Electroless metal deposition is the chemical deposition of a metal or mixture of metals over a catalytic surface by chemical reduction. If a substrate to be plated is not catalytic to metal deposition, the substrate is catalyzed prior to deposition by treatment with a suitable catalyst that renders the surface catalytic to electroless metal deposition.
The catalyst in most common commercial use today comprises the reaction product of a molar excess of stannous tin with palladium ions in hydrochloric acid solution. The reaction product is believed to be a tin-palladium colloid. It is believed that the oxidized tin forms a protective colloid for the palladium and the excess stannous acts as an antioxidant. Colloidal tin-palladium catalysts were first described in U.S. Pat. No. 3,011,920 incorporated herein by reference.
An improvement in colloidal tin-palladium catalysts is disclosed in U.S. Pat. No. 3,904,792 incorporated herein by reference. In this patent, in order to provide catalysts less acidic than those disclosed in the aforesaid U.S. Pat. No. 3,011,920, a portion of the hydrocholoric acid is replaced by a soluble salt of the acid resulting in a more stable catalyst having a pH that can approach about 3.5, but not exceed this value. The catalysts of this patent have also encountered significant commercial success.
The colloidal tin-palladium catalysts have been used in significant quantity since their introduction in about 1958 without change other than the substitution of the salt of the hydrochloric acid for the acid as described above, though during this period and especially since about 1970, considerable efforts have been made to find new and better catalysts. For example, because of the high cost of palladium, considerable effort has been directed toward the development of a non-noble metal catalyst, particularly towards the development of a colloidal copper catalyst. Though functional catalysts from copper are now reported to be in commercial use, it is believed that this use is limited as such catalysts are subject to oxidative attack resulting in loss of stability and/or functionality. In addition, such catalysts are believed to require a high concentration of copper to compensate for the limited catalytic activity of copper relative to palladium. In addition, such catalysts require a highly active copper plating solution of limited stability to compensate for the limited activity of copper as a catalyst.
Another direction that catalyst research has taken is towards the development of a tin free palladium catalyst since the stannous chloride used to reduce the palladium is costly and the oxidized tin requires a separate step of acceleration. Tin free noble metal catalysts, believed to be colloidal, are disclosed in U.S. Pat. No. 4,004,051, also incorporated herein by reference. It is believed that the catalysts of U.S. Pat. No. 4,004,051 have not been used in commerce for any purposes. For example, they are unsuitable for the manufacture of printed circuit boards and multi-layer printed circuits or for plating on plastics because the catalysts are not sufficiently active nor reliable for through-hole plating. Furthermore, these catalysts typically become progressively less active upon standing, and this change in activity renders such catalysts unreliable and impractical for commercial use. One test of the ability of a catalyst to cover a substrate with electroless metal is known as the backlight test (more fully described below). In this test, voids in a copper deposit over a catalyzed surface of a through-hole in a printed circuit board are revealed, if present. It has been found that many of the freshly prepared catalysts of the above referenced patent are incapable of passing the backlight test. With respect to those freshly prepared catalysts tested that were able to meet the requirements of this test, after standing for several days, catalytic activity had decreased and, to the extent that the catalysts were still active, they failed the backlight test.
Though not wishing to bound by theory, it is believed that it is one discovery of the invention described herein that the catalysts of U.S. Pat. No. 4,004,051 are unsuitable for commercial use because the active species comprising the catalysts, believed to be colloidal catalytic metal, are of a particle size too large for adequate colloidal stability. In addition, it is believed that the particles vary considerably with respect to particle size distribution. For example, the patent describes the particles as having a reduced noble metal particle size not exceeding above 0.2 microns (2000 Angstroms). The lower limit for particle size is not given, but based upon the poor performance of these catalysts, it is believed that very small particle size, i.e. less than 500 Angstroms could not be obtained by the methods used to produce the catalysts of said patent. Moreover, it is believed that a catalyst having a reduced catalytic metal particle with a mean particle size approaching 2000 Angstroms will not have sufficient stability for extended commercial use.
Noble metal colloids of small particle size--i.e., 500 Angstroms or less, are believed to be known and used in arts other than in the art of electroless metal plating, but it is believed that such colloids are formed in a non-aqueous solution such as in hydrocarbon solution. Typical of such colloids are disclosed, for example, in U.S. Pat. Nos. 4,059,541 and 4,252,677, both incorporated herein by reference. The inert solvent is believed to be in part responsible for forming and maintaining a colloid of small particle size. However, the inert solvent is expensive to use, many of such solvents would inactivate the catalyst, others might attack plastic substrates and in general, a hydrocarbon solvent would be viewed as undesirable for commercial use. Moreover, to be used as a catalyst for electroless metal deposition, the reduced metal colloid would have to be adsorbed and held on a surface to be plated. It is not known that the reduced noble metal colloids of the aforesaid patent would adsorb onto a surface.